1. Technical Field
The invention relates generally to vibration damping apparatus. More specifically, the invention relates to a vibration damping apparatus used to damp vibrations produced when drilling a well.
2. Background Art
Typically, drilling systems used by the oil and gas industry to drill wells in earth formations include a drilling rig used to turn a drill string which extends downward into a wellbore. Connected to the end of the drill string is a bottom hole assembly (xe2x80x9cBHAxe2x80x9d) that may include, for example, a drill bit, a positive displacement motor, measurement while drilling (xe2x80x9cMWDxe2x80x9d) tools, and logging while drilling (xe2x80x9cLWDxe2x80x9d) tools, among other tools.
Drilling activity typically involves applying an axial load to the drill bit when the bit is in contact with the formation at the bottom of the wellbore, while rotating the bit. When the bit drills the formation, it produces vibrations and oscillations of the BHA and of drillpipe located above the BHA. These vibrations and oscillations are undesirable because they may cause fatigue and, ultimately, structural failure of elements of the BHA, the drillpipe, or the drill bit. Moreover, the vibrations may have damaging effects on electronic instrumentation present in the MWD and LWD tools.
Many efforts have been made to design mechanical devices to reduce oscillations, shocks, and vibrations. Some of the devices use reciprocating mandrels in combination with a compressible fluid filled chamber to absorb shocks and dampen vibrations, such as the device disclosed in U.S. Pat. No. 4,439,167 issued to Bishop et. al. Other devices use a plurality of resilient elastomer elements or belleville springs to absorb axial shocks, such as the floating sub disclosed in U.S. Pat. No. 4,844,181 issued to Bassinger. Another class of devices uses floating pistons and compressible fluid filled chambers to absorb axial vibrations, such as the device disclosed in U.S. Pat. No. 4,901,806 issued to Forrest. Another class of devices uses a helically splined mandrel or annular springs to absorb vibrations, such as the drill string shock absorber disclosed in U.S. Pat. No.3,947,008 issued to Mullins. U.S. Pat. No. 3,265,091 issued to De Jarnett discloses another device for absorbing vibrations that includes a drill pipe having an inner steel tube and an outer steel tube. The annular space between the tubes is filled with a fluid of preselected density that acts to damp or absorb vibrations.
One aspect of the invention is a vibration damping apparatus including an annular housing, a cavity between an internal diameter and an external diameter of the housing, and a substantially solid vibration damping material disposed in the cavity. The substantially solid vibration damping material has a density that is greater than a density of a material from which the housing is formed.
In another aspect of the invention, the internal diameter and the external diameter of the housing are selected so as to maximize an amount of vibration damping material with respect to a selected tensile strength for the housing.
In another aspect of the invention, the internal diameter and the external diameter of the housing are selected so as to maximize an amount of vibration damping material with respect to a selected bending strength for the housing.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.